Users and groups are used on GNU/Linux for access control — that is, to control access to the system's files, directories, and peripherals. Linux offers relatively simple/coarse access control mechanisms by default. For more advanced options, see ACL and LDAP Authentication.

Contents

Overview

A user is anyone who uses a computer. In this case, we are describing the names which represent those users. It may be Mary or Bill, and they may use the names Dragonlady or Pirate in place of their real name. All that matters is that the computer has a name for each account it creates, and it is this name by which a person gains access to use the computer. Some system services also run using restricted or privileged user accounts.

Managing users is done for the purpose of security by limiting access in certain specific ways.

Any individual may have more than one account, as long as they use a different name for each account they create. Further, there are some reserved names which may not be used such as "root".

Users may be grouped together into a "group," and users may choose to join an existing group to utilize the privileged access it grants.

Note: The beginner should use these tools carefully and stay away from having anything to do with any other existing user account, other than their own.

Permissions and ownership

The UNIX operating system crystallizes a couple of unifying ideas and concepts that shaped its design, user interface, culture and evolution. One of the most important of these is probably the mantra: "everything is a file," widely regarded as one of the defining points of UNIX.

This key design principle consists of providing a unified paradigm for accessing a wide range of input/output resources: documents, directories, hard-drives, CD-ROMs, modems, keyboards, printers, monitors, terminals and even some inter-process and network communications. The trick is to provide a common abstraction for all of these resources, each of which the UNIX fathers called a "file." Since every "file" is exposed through the same API, you can use the same set of basic commands to read/write to a disk, keyboard, document or network device.

A fundamental and very powerful, consistent abstraction provided in UNIX and compatible operating systems is the file abstraction. Many OS services and device interfaces are implemented to provide a file or file system metaphor to applications. This enables new uses for, and greatly increases the power of, existing applications — simple tools designed with specific uses in mind can, with UNIX file abstractions, be used in novel ways. A simple tool, such as cat, designed to read one or more files and output the contents to standard output, can be used to read from I/O devices through special device files, typically found under the /dev directory. On many systems, audio recording and playback can be done simply with the commands, "cat /dev/audio > myfile" and "cat myfile > /dev/audio," respectively.

Every file on a GNU/Linux system is owned by a user and a group. In addition, there are three types of access permissions: read, write, and execute. Different access permissions can be applied to a file's owning user, owning group, and others (those without ownership). One can determine a file's owners and permissions by viewing the long listing format of the ls command:

The first column displays the file's permissions (for example, the file kernel26.img has permissions -rw-r--r--). The third and fourth columns display the file's owning user and group, respectively. In this example, all files are owned by the root user and the root group.

$ ls /media/ -l

total 16
drwxrwx--- 1 root vboxsf 16384 Jan 29 11:02 sf_Shared

In this example, the sf_Shared directory is owned by the root user and the vboxsf group. It is also possible to determine a file's owners and permissions using the stat command:

Owning user:

$ stat -c %U /media/sf_Shared/

root

Owning group:

$ stat -c %G /media/sf_Shared/

vboxsf

Access rights:

$ stat -c %A /media/sf_Shared/

drwxrwx---

Access permissions are displayed in three groups of characters, representing the permissions of the owning user, owning group, and others, respectively. For example, the characters -rw-r--r-- indicate that the file's owner has read and write permission, but not execute (rw-), whilst users belonging to the owning group and other users have only read permission (r-- and r--). Meanwhile, the characters drwxrwx--- indicate that the file's owner and users belonging to the owning group all have read, write, and execute permissions (rwx and rwx), whilst other users are denied access (---). The first character represents the file's type.

List files owned by a user or group with the find command:

# find / -group [group]

# find / -user [user]

A file's owning user and group can be changed with the chown (change owner) command. A file's access permissions can be changed with the chmod (change mode) command.

User management

Local user information is stored in the /etc/passwd file. To list all user accounts on the system:

$ cat /etc/passwd

There is one line per account, and each is of the format:

account:password:UID:GID:GECOS:directory:shell

where:

account is the user name

password is the user password

UID is the numerical user ID

GID is the numerical primary group ID for the user

GECOS is an optional field used for informational purposes; usually it contains the full user name

directory is the user's $HOME directory

shell is the user command interpreter (defaults to /bin/sh)

Note: Arch Linux uses shadowed passwords. The passwd file is world-readable, so storing passwords (hashed or otherwise) in this file would be insecure. Instead, the password field will contain a placeholder character (x) indicating that the hashed password is saved in the access-restricted file /etc/shadow.

To list users currently logged on the system, the who command can be used.

-m creates the user home directory as /home/[username]; within his home directory, a non-root user can write files, delete them, install programs, and so on.

-g defines the group name or number of the user's initial login group; the group name must exist; if a group number is provided, it must refer to an already existing group; if not specified, the behavior of useradd will depend on the USERGROUPS_ENAB variable contained in /etc/login.defs.

-G introduces a list of supplementary groups which the user is also a member of; each group is separated from the next by a comma, with no intervening spaces; the default is for the user to belong only to the initial group.

-s defines the path and filename of the user's default login shell; Arch Linux init scripts use Bash; after the boot process is complete, the default login shell is the one specified here; ensure the chosen shell package is installed if choosing something other than Bash.

A typical desktop system example, adding a user named archie specifying bash as the login shell:

To enter user information for the GECOS field (e.g. the full user name), type:

# chfn [username]

(this way chfn runs in interactive mode).

To specify the user's password, type:

# passwd [username]

An interactive tool is available for adding users:

# adduser

adduser asks common questions about the user with sane defaults and constructs a useradd command to run. It also changes finger information and sets the password, thus carrying out also the job of chfn and passwd.

User accounts may be deleted with the userdel command.

# userdel -r [username]

The -r option specifies that the user's home directory and mail spool should also be deleted.

Group management

/etc/group is the file that defines the groups on the system (man group for details).

Display group membership with the groups command:

$ groups [user]

If user is omitted, the current user's group names are displayed.

The id command provides additional detail, such as the user's UID and associated GIDs:

$ id [user]

To list all groups on the system:

$ cat /etc/group

Create new groups with the groupadd command:

# groupadd [group]

Add users to a group with the gpasswd command:

# gpasswd -a [user] [group]

To delete existing groups:

# groupdel [group]

To remove users from a group:

# gpasswd -d [user] [group]

If the user is currently logged in, he/she must log out and in again for the change to have effect.

Group list

User groups

Workstation/desktop users often add their non-root user to some of following groups to allow access to peripherals and other hardware and facilitate system administration:

Group

Affected files

Purpose

audio

/dev/audio, /dev/snd/*, /dev/rtc0

Access to sound hardware (requirement is imposed by both ALSA and OSS).